This invention relates to a wireless communication system and to a method of operating such a system.
An important effect in wireless communication and especially indoor radio data communication is fading. This is the phenomenon which occurs when multiple copies of an omnidirected transmitted signal arrive at a receiver antenna via different paths, having different propagation delays. The signals will add together, but with different relative phases with respect to each other, which may result in an increased signal level, but may also result in a much lower level signal when multiple received signals cancel each other out, depending on the relative phases of the signals.
To deal with this phenomenon, wireless communication systems need to be designed with significant margins in the receiver dynamic range, such that the added attenuation as a result of a fade can be overcome. The probability of a signal level at a given distance between the transmitter and the receiver is given by a so-called Rayleigh attenuation model. Maintaining a given margin for this effect results in a so-called outage probability, which is the probability that a fade is so deep that the signal cannot be received. The outage probability can be significantly reduced if more than one antenna is used for reception of the signal, as long as the received signal levels on the antennas are independent or uncorrelated.
Different methodologies can be used to combine the signals received over the antennas. One of the methods which can be implemented with only a single radio receiver is the so-called selection diversity system, which selects the best antenna based on the measured signal quality. The resulting outage probability of such a system is the product of the individual probabilities, and is therefore considerably reduced.
Such a selection diversity system is described, for example, in U.S. Pat. No. 5,131,006, which discloses a wireless local area network including a plurality of stations adapted to communicate with one another over a common wireless communication channel. Each station has two antennas, which are polarized in different directions, such as at right angles to each other. Packets transmitted in the network include an initial training signal portion which extends over a plurality of antenna selection periods, referred to as antenna slot periods. During the receipt of a training signal at a station, the station alternates between its antennas for a plurality (e.g. four) of successive antenna slot periods, the received signal quality for the respective antennas is measured, and the station selects the antenna giving the better signal quality for receipt of the subsequent portion of the transmitted packet. This procedure has the disadvantage that a long training signal is needed for each transmitted packet to enable the antenna selection procedure to be effected.